I. Field of the Invention
This invention relates, in general, to brazing filler metals and, more specifically, to filler metal compositions used to form strong joints. In particular, the invention relates to novel filler metals used to form strong joints between parts with gaps substantially greater than heretofore permitted with known filler metals having melting points of about 1,775.degree. F. The invention further relates to a vacuum brazing process utilizing said filler metals.
II. Description of the Prior Art
Brazing consists of joining base metal surfaces by fusing a filler metal, having a lower melting point than the subject base metal, without appreciable fusion of the base metal surfaces themselves. For brazing, a flux may be applied to the subject base metal surfaces either prior to or simultaneously with the filler metal.
A satisfactory brazing flux flows at a temperature somewhat below the melting point of the filler metal; adheres to or wets the base metal surfaces; facilitates the flow and wetting of the filler metal over the subject base metal surfaces generally by reducing the surface tension of the molten filler metal; removes any oxide coating or other adherent foreign matter present on the subject base metal surfaces without appreciably attacking the base metal surfaces; inhibits re-oxidation of the subject base metal surfaces; and is capable of ready displacement by liquid filler metal either leaving no residue or leaving a readily removable, relatively inert residue after completion of the brazing.
Furnace brazing in a vacuum with the use of no flux offers several advantages. For example, the possibility of flux inclusions are eliminated and, accordingly, blind cavities, tortuous paths, and small passageways can be designed into the assembly without regard to flux removal or entrapment after brazing. In addition, fluxless vacuum brazing eliminates the cost of flux and its application, the need for cleaning the assembly after brazing, and potential corrosion of equipment and pollution of air and water by flux residues or flux reaction products.
Nickel-base, copper-base, gold-base, palladium-base, and a few silver-base filler metals are commonly used in vacuum furnace brazing. Apart from compatability with the base metal, filler metals are invariably selected for corrosion resistance in specific media and suitability for service at known operating temperatures.
It has become increasingly important, especially in high temperature aircraft applications, such as, for example, in turbine engine components, to use materials for structural applications that are capable of withstanding high temperatures and corrosive attacks normally associated therewith. Stainless steels and the so-called superalloys, such as nickel-base superalloys, have been employed, of course, where possible to meet requirements of high strength-to weight ratios, corrosion resistance, etc. at elevated temperatures. The greatest impediment to efficient use of these materials, however, has been the difficulty of obtaining satisfactory joints between parts made of such materials.
The amount of filler metal applied to a joint area must usually be carefully controlled to avoid flowing into areas where it is neither needed nor wanted, as well as to avoid interalloying of base metal and filler metal which is often harmful to joint strength. The brazing alloy should be compatible with the optimum heat treatment temperature of the base metal. Known filler metals for nickel-base alloys also do not usually give good wetting, by limited flow, at brazing temperatures of about 1,775.degree. F. so that joints are sealed without filler material flowing into internal passage of the components. In addition, known filler metals do not have proper wetting and flow characteristics at a brazing temperature of about 1,775.degree. F. while also providing good high temperature corrosion and abrasion resistance. Alloys giving the desired limited flow usually melt at higher temperatures, thus interferring with the base metal.
It is, therefore, an object of this invention to provide a brazing filler metal composition which is devoid of the above-noted disadvantages.
It is another object of this invention to provide a brazing filler metal composition for use in forming strong joints between parts with gaps, substantially greater than those commonly permitted with known nickel-base filler metals.
It is a further object of this invention to provide a brazing filler metal composition which has desired properties for use in forming strong joints between high temperature superalloys, such as those used in turbine engine high temperature components.
It is still another object of this invention to provide brazing filler metals which wet well and yet have very limited flow at a brazing temperature of about 1,775.degree. F.
It is yet another object of this invention to provide brazing filler metals which allow brazing and heat treating in a single cycle with base metals requiring solution treatment at a temperature of about 1,750.degree. F. to obtain optimum strength.
It is another further object of this invention to provide brazing filler metals which provide very good high temperature and corrosion resistance.
It is still another further object of this invention to provide a vacuum brazing process utilizing novel nickel-base filler compositions.